In October 2011, during a shadow ministerial reshuffle, Catherine McKinnell was made shadow children's minister, shadowing Tim Loughton. In that post she criticised the adoption process as too slow and called for immediate improvements in support for social workers and family courts to speed up the process.[9][10] She also accused the government of doing too little to help children for whom adoption was not suitable and following this, requested a guarantee that the government would give priority to placing children in "happy homes|.[11]

In 2012, after the resignation of Peter Hain, she was then moved to become Shadow Exchequer Secretary to the Treasury, replacing Owen Smith.

McKinnell backed a campaign by ActionAid on international tax laws[12] and tabled amendments to the Budget which would have required the government to monitor the impact on developing countries of changes to so-called Controlled Foreign Companies regulations. She said, "It seems a false economy to invest ... in changes that will undermine the very progress towards which our international aid money, which increases year on year, is going."[13] In June 2012, McKinnell publicly criticised Take That singer Gary Barlow following newspaper allegations of tax avoidance made against him. McKinnell agreed that Barlow should consider returning his recently awarded OBE if allegations of tax avoidance were proven "because it doesn't send out the right messages to ordinary people who are paying their fair share of tax".[14]

She was made Shadow Attorney General in September 2015 by Labour leader Jeremy Corbyn, but resigned in January 2016, citing party infighting, family reasons and the ability to speak in parliament beyond her legal portfolio.[15]

Mckinnell has been a prominent campaigner for the Women Against State Pension Inequality campaign, who, following the acceleration of the equalisation of the State Pension Age, have argued that the acceleration has happened too quickly and left female pensioners uncertain.[16] Mckinnell was also made Vice Chair of the recently established All-Party Parliamentary Group on the WASPI campaign.[17]